1. Field of the Invention
The present invention generally relates to optical pickup units and information recording and reproduction apparatuses, and more particularly to an optical pickup unit recording information on and reproducing information from information recording media and an information recording and reproduction apparatus including such an optical pickup unit.
2. Description of the Related Art
Disk-type optical information recording media widely used nowadays are compact disks (CDs) such as CD-ROMs, CD-Rs, and CD-RWs and digital versatile disks (DVDs) such as DVD-ROMs, DVD-Rs, and DVD-RWs. Some CDs have realized a recording density of 650 MB. The DVDs are larger in capacity than the CDs, but have yet to satisfy the demands of users in terms of capacity. In this context, a so-called multilayer disk, a disk formed of a plurality of recording layers instead of a single recording layer, has been developed for achieving higher recording density.
In such an information recording medium of multiple recording layers, the recording layers are required to be separated from each other by tens of micrometers (μm) or more so that information may be recorded on and reproduced from each recording layer independently. However, distances from an objective lens to the recording layers are different so that spherical aberration occurs in recording layers out of an optimum position. That is, in such recording layers, the position of the focus of marginal rays (rays entering the periphery of a lens away from the optical axis thereof) entering a lens 90 shown in FIG. 1 is deviated in the direction of the optical axis from the position of the focus of paraxial rays (rays entering the central part of a lens) entering the lens 90.
FIG. 2 is a diagram for illustrating focusing of a light beam in the case of reproducing information from a multilayer disk by a single conventional optical pickup unit. In the case of reproducing information from a first recording layer 101a of a multilayer disk 101, the light beam is focused on the first recording layer 101a through an objective lens of the optical pickup unit at a position indicated by 100a as shown on the left side in FIG. 2. In the case of reproducing information from an nth recording layer 101n, which is farther away from a disk substrate surface 101s than the first recording layer 101a is, the light beam is focused on the nth recording layer 101n through the objective lens at a position indicated by 100b as shown on the right in FIG. 2.
If the optical pickup unit is optimized for focusing the light beam into a spot on the first recording layer 101a to reproduce the information therefrom, the optical pickup unit has no problem in reproducing the information from the first recording layer 101a. However, when the objective lens of the optical pickup unit gets closer to the disk substrate surface 101s of the multilayer disk 101 to reproduce the information from the nth recording layer 101n, and the light beam is focused into a spot on the nth recording layer 101n, spherical aberration occurs due to an interlayer thickness between the first and nth recording layers 101a and 101n. As a result, the spot formed on the nth recording layer 101n is larger in diameter than the spot formed on the first recording layer 101a as shown in FIG. 2.
In order to solve this problem, it is necessary to develop a new optical head control technology of performing aberration correction by measuring the amount of spherical aberration of a beam spot. Japanese Laid-Open Patent Application No. 2000-155979 discloses an aberration detection device as means for solving this problem.
FIG. 3 is a diagram showing a configuration of the aberration detection device. As shown in FIG. 3, a light beam emitted from a light source 201 and reflected from an optical disk 206 is split by a half mirror 202 to be divided into a light beam passing through a specific region and a light beam passing through the other regions by a hologram 209. The light beam passing through the specific region is deflected by the hologram 209 to be received by a plurality of photodetectors 207 so that the photodetectors 207 obtain respective signals. The obtained signals are compared so that an aberration is detected. The detected aberration is transmitted via an aberration signal processing circuit 208 to an aberration correction device 204 so that the aberration correction device 204 can be driven in real time based on the aberration so as to reduce the aberration of the optical system. In FIG. 3, reference numerals 203 and 205 denote a collimator lens and an objective lens, respectively.
The assembly conditions of a light-receiving element in the optical pickup unit are extremely strict so that the light-receiving element is required to be provided with an accuracy of a few micrometers or less. This reduces yield rate, thereby affecting the cost to a considerable extent. Further, the aging and temperature characteristics of the light-receiving element are subject to change. Therefore, it is desirable that a light-receiving element pattern be simple. However, according to the technology disclosed in Japanese Laid-Open Patent Application No. 2000-155979, an incident light beam for aberration detection is split into a plurality of beams by a hologram so that a plurality of light-receiving elements detect a given one of the split beams. Such a configuration, where each split beam is focused into a small spot and a plurality of light-receiving elements detect a given one of the split beams, is complicated and may impair the stability of the optical pickup unit. This may reduce yield rate, thereby incurring an increase in cost.
The optical pickup unit is known as a device for recording information on and reproducing recorded information from an information recording medium. The ratio of the intensity of a light beam focused onto the information recording medium for recording to that for reproduction ranges from 5:1 to 15:1. Generally, the emission power of a light source switches between recording time and reproduction time substantially in accordance with the ratio. However, in some semiconductor lasers, noise characteristics worsen as an output lowers.
That is, part of a light beam emitted from a semiconductor laser returns to the semiconductor laser as a returning light. A different resonator other than the semiconductor laser is formed between the returning light and the information recording medium. As a result, the state of oscillation of the semiconductor laser becomes unstable so that the output of the semiconductor laser includes noise. Further, noise is also generated in the output of the semiconductor laser by the operation of the semiconductor laser or a variation in environmental temperature. When the light-emission power of the semiconductor laser is far above a threshold current level to reach tens of milliwatts (mW), the light emission of the semiconductor laser is stable, being hardly affected by disturbances. However, when the light-emission power of the semiconductor laser is around the threshold current level, the light-emission power is affected by disturbances including those caused by the above-described returning light, so that variations are caused in the light-emission power.
Accordingly, in the case of recording information on (writing information to) the information recording medium or erasing information recorded thereon, the light emission of the semiconductor laser is hardly affected by disturbances since the light-emission power of the semiconductor laser is far above the threshold current level. However, in the case of reproducing (reading out) information from the information recording medium, the light-emission power of the semiconductor laser is normally set to a low level so that the semiconductor laser emits the laser beam with a power of a few milliwatts slightly over the threshold current level, for instance, a power of five milliwatts. In this case, therefore, the semiconductor laser is especially subject to the returning light to be unstable in emitting the laser beam. Accordingly, the signal is deteriorated by noise caused in the output of the semiconductor laser.
Japanese Laid-Open Patent Application No. 9-27141 discloses an optical pickup unit to solve this problem. According to this optical pickup unit, an electro-optical element capable of controlling transmittance of light is provided in an optical path from a light source to a recording medium. The electro-optical element controls transmittance for light emitted from the light source to a low rate (value) at a time of reproducing information, and to a high rate (value) at a time of recording information.
Further, in an optical pickup unit, light emitted from a semiconductor laser is focused onto a surface of an information recording medium through a focus optical system, and a reflected light from the information recording medium is directed through a detection optical system to a light-receiving element. Generally, the detection signal (electric current signal) of the light-receiving element of the optical pickup unit is converted into a voltage signal by a current-voltage conversion amplifier housed in the optical pickup unit to be output to-a signal processing circuit.
If the amplitude level of the signal output through the current-voltage conversion amplifier is too low, a problem is caused in information reproduction. Therefore, such a configuration is employed that the gain of the current-voltage conversion amplifier is switchable so that the output amplitude level thereof falls within a proper range. However, in recent years, it has been required for the optical pickup unit to accommodate a plurality of conditions so as to be suitable for a variety of types of information recording media and various recording and reproduction conditions. Accordingly, in order to perform information recording, reproduction, and erasure in compliance with a plurality of types of optical disks, such as a CD-RW (compact disk rewritable), a CD-DA (compact disk digital audio), a CD-ROM, and a CD-R (compact disk recordable), as performed by an optical information recording and reproduction apparatus disclosed in Japanese Laid-Open Patent Application No. 10-255301, it is necessary to make such a gain switching circuit shown in FIG. 4 suitable for more combination patterns of disk types and light powers. Consequently, the number of resistors employed in the gain switching circuit increases so that the speed of response of a signal is reduced.
In addition, recently, it has been proposed to increase the numerical aperture (NA) of an objective lens for focusing a light beam onto an information recording medium so as to achieve high recording density. This is because the diameter of a beam spot can be reduced by using an objective lens of a large NA. However, as the NA increases, the rate of increase of aberration also increases. That is, spherical aberration, whose primary cause is a substrate thickness error in the information recording medium, is proportional to the NA to the fourth power, and coma, whose primary cause is the inclination of the information recording medium to the optical axis, is proportional to the cube of the NA.
Japanese Laid-Open Patent Application Nos. 10-20263, 9-128785, 11-259892, and 2000-155979 disclose techniques to correct and detect wavefront aberration (spherical aberration, coma, and astigmatism), which techniques are known as prior art for solving the above-described problems.
However, optical pickup units having such configurations as disclosed in the above-described references need to have their costs reduced by component sharing and their assembly processes simplified.